What Is Rennet? Animal vs Microbial (Cheese, Explained)

You pick up a block of cheddar labeled “suitable for vegetarians,” and it seems straightforward. Then you grab a wedge of Parmesan, and there is no such label. The difference comes down to rennet, the ingredient that turns milk into cheese. Whether a cheese is vegetarian often depends on which kind of rennet was used.

Key Takeaways

• Rennet is the enzyme preparation that coagulates milk into curds and whey. The key enzyme is chymosin.
• Traditional animal rennet comes from the stomachs of calves, and is not vegetarian.
• Microbial, vegetable and fermentation-produced (FPC) rennet are vegetarian alternatives.
• Parmigiano-Reggiano and Grana Padano are required by their production rules to use animal rennet, so they are never vegetarian.

TL;DR: Rennet is the enzyme that curdles milk into cheese. Animal rennet comes from calf stomachs and is not vegetarian. Microbial, vegetable and fermentation-produced rennet are the vegetarian-friendly options.

What rennet does

Milk is mostly water, with protein and fat suspended in it. To make cheese, you separate the solid part (curds) from the liquid part (whey), and rennet is what triggers that separation. The active enzyme in rennet is chymosin, which cuts a specific milk protein so it clumps together (ScienceDirect, retrieved 2026-08-25). Add rennet to warm milk, and within minutes you get a solid gel that can be cut, drained and pressed into cheese.

That one step is why rennet matters. Every cheese uses it, but not every rennet is the same.

The four types of rennet

Animal rennet is the original method. It is extracted from the fourth stomach of young calves, where chymosin naturally occurs to help them digest milk. Because it is a by-product of the veal industry, most vegetarians avoid it.

Microbial rennet comes from microorganisms that produce similar enzymes, and vegetable rennet from certain plants. Both coagulate milk without any animal source. FPC is chymosin made by engineered microbes in a fermenter. It is chemically identical to calf chymosin and is now the most widely used rennet in industrial cheesemaking.

The Parmesan problem

Some cheeses can never be vegetarian, because the rules say so. Parmigiano-Reggiano and Grana Padano are “Grana” cheeses produced under protected, traditional specifications, and their production requires animal rennet (ScienceDirect, retrieved 2026-08-25). The same applies to Pecorino Romano and several other hard Italian cheeses. If a recipe or product says “Parmesan,” assume animal rennet unless it is a vegetarian imitation.

How to check a label

1. Look for “suitable for vegetarians,” “microbial rennet,” or “vegetable rennet.”
2. If it only says “rennet” or “enzymes,” assume animal unless the brand states otherwise.
3. For hard Italian grana cheeses, assume animal rennet by default.
4. For strict vegans, no cheese made with any rennet is vegan. Choose plant-based cheese instead.

Rennet is one of several hidden animal ingredients in dairy. See our E-numbers cheat sheet for the rest, including whey, casein and lactose.

The bottom line

Rennet decides whether a cheese is vegetarian. If you care, check for the “suitable for vegetarians” mark or a stated microbial or vegetable rennet. And if it is Parmesan, it is animal rennet by definition.

Check before you eat. The Verify Veg app scans a barcode or reads an ingredient list from a photo and flags ingredients against your diet (vegan, vegetarian, halal, kosher, Jain and more). Download on the App Store - Android is coming soon.

Written by Samir Narain, founder of Verify Veg. Verify Veg helps people check packaged food against their diet by scanning barcodes and reading ingredient lists from photos.

Note: Verify Veg provides ingredient-based compatibility checks, not certification. Ingredient sources can vary by manufacturer, so always confirm with the brand or vendor.